Melt butter in a saucepan. Add brown sugar. Blend thoroughly and spread evenly in the bottom of a 9x13-inch cake pan. Arrange pineapple slices over the surface of mixture. Cover with the cake batter (it will be very dense; use a spatula to spread it throughout the cake pan). Bake for 50 to 60 minutes.

Let the cake cool for 15 minutes, then slide a knife around the outside edges of the cake to loosen it from the pan. Place a serving plate over the top of the cake pan and turn over, flipping the cake onto the plate.